This study examins the effects of an experimental drug, Modafinil, in the treatment of Narcolepsy. The study is being carried out in approximately 26 sites around the US. Subjects with a history of Narcolepsy are screened to assume absense of other significant medical problems and drug-free status. They are then brought into the CRC and sleep lab for overnight sleep studies and daytime tests to confirm the diagnosis of Narcolepsy and to establish the level of baseline functioning. Subjects are then treated in a nine week, double-blind, placebo-controlled paradigm with Modafinil, 200mg or 400mg. Follow up studies of sleep and daytime functioning are carried out at two other times during the protocol.